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                 THIRD AMENDMENT TO REVOLVING CREDIT AGREEMENT
                 ---------------------------------------------

    THIS THIRD AMENDMENT TO REVOLVING CREDIT AGREEMENT (this "AMENDMENT"),
dated as of October 10, 1997, is entered into by and between CHIRON
CORPORATION (the "BORROWER") and BANK OF AMERICA NATIONAL TRUST AND SAVINGS
ASSOCIATION (the "BANK").

                                   RECITALS
                                   --------

    A.   The Borrower and the Bank are parties to a Revolving Credit 
Agreement dated as of July 12, 1996, as amended by a First Amendment to 
Revolving Credit Agreement dated as of July 11, 1997, and a Second Amendment 
to Revolving Credit agreement dated as of August 18, 1997 (as so amended, the 
"CREDIT AGREEMENT") pursuant to which the Bank has extended certain credit 
facilities to the Borrower.

    B.   The Borrower has requested that the Bank agree to a certain
amendment of the Credit Agreement.

    C.   The Bank is willing to amend the Credit Agreement, subject to the
terms and conditions of this Amendment.

    NOW, THEREFORE, for valuable consideration, the receipt and adequacy
of which are hereby acknowledged, the parties hereto hereby agree as
follows:

    1.   DEFINED TERMS.  Unless otherwise defined herein, capitalized
terms used herein shall have the meanings, if any, assigned to them in the
Credit Agreement.

    2.   AMENDMENT TO CREDIT AGREEMENT.  Paragraph 1 of the Credit Agreement 
shall be amended and restated in its entirety so as to read as follows:

         1.   THE ADVANCES.  Subject to and upon the terms and conditions
         set forth herein, the Bank agrees to make advances (the
         "ADVANCES") to the Borrower at any time and from time to time
         prior to December 12, 1997 (the "EXPIRY DATE"); provided, however,
         that the aggregate principal amount of Advances outstanding shall
         at no time exceed U.S. $100,000,000 (the "COMMITMENT").

    3.   REPRESENTATIONS AND WARRANTIES.  The Borrower hereby represents
and warrants to the Bank as follows:

         (a)  No Event of Default or condition, event or act which, with
the giving of notice or lapse of time, or both, would constitute an Event
of Default has occurred and is continuing.

         (b)  The execution, delivery and performance by the Borrower of
this Amendment have been duly authorized by all

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necessary corporate and other action and do not and will not require any
registration with, consent or approval of, notice to or action by, any
person (including any governmental authority) in order to be effective and
enforceable.  The Credit Agreement as amended by this Amendment constitutes
the legal, valid and binding obligations of the Borrower, enforceable
against it in accordance with its respective terms, without defense,
counterclaim or offset.

         (c)  All representations and warranties of the Borrower contained
in the Credit Agreement are true and correct.

         (d)  The Borrower is entering into this Amendment on the basis of
its own investigation and for its own reasons, without reliance upon the
Bank or any other person.

    4.   EFFECTIVE DATE. This Amendment will become effective as of 
October 10, 1997 (the "EFFECTIVE DATE"), PROVIDED THAT the Bank has
received from the Borrower a duly executed original (or, if elected by the
Bank, an executed facsimile copy) of this Amendment.

    5.   RESERVATION OF RIGHTS.  The Borrower acknowledges and agrees that
the execution and delivery by the Bank of this Amendment shall not be
deemed to create a course of dealing or otherwise obligate the Bank to
execute similar amendments under the same or similar circumstances in the
future.

    6.   MISCELLANEOUS.

         (a)  Except as herein expressly amended, all terms, covenants and
provisions of the Credit Agreement are and shall remain in full force and
effect and all references therein to such Credit Agreement shall henceforth
refer to the Credit Agreement as amended by this Amendment.  This Amendment
shall be deemed incorporated into, and a part of, the Credit Agreement.

         (b)  This Amendment shall be binding upon and inure to the
benefit of the parties hereto and thereto and their respective successors
and assigns.  No third party beneficiaries are intended in connection with
this Amendment.

         (c)  This Amendment shall be governed by and construed in
accordance with the law of the State of California.

         (d)  This Amendment may be executed in any number of
counterparts, each of which shall be deemed an original, but all such
counterparts together shall constitute but one and the same instrument. 
Each of the parties hereto understands and agrees

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that this document (and any other document required herein) may be
delivered by any party thereto either in the form of an executed original
or an executed original sent by facsimile transmission to be followed
promptly by mailing of a hard copy original, and that receipt by the Bank
of a facsimile transmitted document purportedly bearing the signature of
the Borrower shall bind the Borrower with the same force and effect as the
delivery of a hard copy original.  Any failure by the Bank to receive the
hard copy executed original of such document shall not diminish the binding
effect of receipt of the facsimile transmitted executed original of such
document which hard copy page was not received by the Bank.

         (e)  This Amendment, together with the Credit Agreement, contains
the entire and exclusive agreement of the parties hereto with reference to
the matters discussed herein and therein.  This Amendment supersedes all
prior drafts and communications with respect thereto.

         (f)  If any term or provision of this Amendment shall be deemed
prohibited by or invalid under any applicable law, such provision shall be
invalidated without affecting the remaining provisions of this Amendment or
the Credit Agreement, respectively.

         (g)  Borrower covenants to pay to or reimburse the Bank, upon
demand, for all reasonable costs and expenses (including allocated costs of
in-house counsel) incurred in connection with the development, preparation,
negotiation, execution and delivery of this Amendment.


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IN WITNESS WHEREOF, the parties hereto have executed and delivered this
Amendment as of the date first above written.



                                       CHIRON CORPORATION

                                       By: /s/ JE Kent
                                          -------------------
                                       Title: VP, Treasurer 
                       

                                       BANK OF AMERICA NATIONAL TRUST
                                       AND SAVINGS ASSOCIATION

                                       By: /s/ K McMahon
                                          -------------------
                                       Title: Managing Director


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